Clean up VirtualEnv API

[nickjalbert]

I started some clean up work on the VirtualEnv API in #296, but we can probably make it better. In general, I imagine the primary public interface could look like the following:

class VirtualEnv:
    def activate(self):
         pass

    def deactivate(self):
         pass

    def install_requirements_file(self, file_path):
         pass

    # To support setup.py ala #275 
    def install_requirement(self, requirement_name):
         pass

And then some bonus class methods and helper methods. I don't think we're too far from this after #296 is merged, but it will probably be good to do another pass before tackling #275.

[andyk]

@nickjalbert is this done now (i.e., can we close this issue) as of #306 being merged which addressed #275? I think that at least activate() and deactivate() now are there, not sure if you have other changes you want to make to the way we are handling requirements files?

[nickjalbert]

Yeah, let's close this for now. I still think there's some work to do on virtual_env.py, but nothing super low-hanging.